1. Field of the Invention
The present invention concerns a method of determining the rotor position of synchronous motors, in particular multi-phase synchronous motors, for regulating the synchronous motors to optimum efficiency, as set forth pursuant to the present disclosure, a method of detecting a zero passage of a stator current changing in sign of a synchronous motor, as set forth in the disclosure.
2. Discussion of the Prior Art
Electronically commutated synchronous machines or motors which are operated on a dc voltage network or which are converter-fed are known from the literature and various situations of use in a practical context. Synchronous motors are also increasingly coming into use in the sector of low rotary speed dynamics as for example in connection with pumps, condensers or washing machines. Besides the high level of starting torque, synchronous motors have the advantage over asynchronous motors inter alia that they can be operated with larger air gap tolerances, thereby affording structural advantages such as for example the direct drive for the washing drum in washing machines or pumps and condensers with a wet rotor.
It is known that, in the case of synchronous motors, an optimum torque and thus an optimum level of efficiency are achieved if the vector of the magnetic flux .phi..sub.R produced by the rotor is perpendicular to the vector of the magnetic flux .phi..sub.S generated by the respective stator winding, that is to say if the magnetic field of the rotor is oriented in perpendicular relationship to the magnetic field of the respective stator winding. That arises out of the fact that the torque vector T is proportional to .phi..sub.R.times..phi..sub.S or the magnitude of the torque vector T is proportional to sin.alpha., wherein .alpha. is the spatial setting angle between the two magnetic fluxes .phi..sub.R and .phi..sub.S. In this case the rotor of the synchronous motor is so-to-speak pulled along by the rotating stator rotary field.
As the magnetic flux .phi..sub.R generated by the rotor is determined directly by the position of the rotor, synchronous motors can be regulated for example by detection of the position of the rotor in relation to the rotating stator field. In that respect, it is known from the state of the art to provide at the rotor shaft of the synchronous motor sensors which establish the position of the rotor at any moment in time. A regulating apparatus of that kind is known for example from DE-A1 195 27 982 in which detection of the position, speed of rotation and/or direction of rotation of the rotor is effected by the use of stationarily mounted, magnetosensitive sensors, the measurement signals of which are fed to the electronic control system.
It is also known to manage without sensors of that kind when regulating synchronous motors. If the stator winding is acted upon by a so-called gappy current, that is to say in particular a current of staircase-shaped or rectangular configuration with phases in which the current is constantly zero, it is possible, in those so-called current gaps, to detect the voltage which is induced by the rotation of the rotor in the stator winding and which is also referred to briefly as the emf as potential applied to the corresponding motor terminal, and to obtain therefrom information about the position of the rotor. Regulation of the synchronous motor is then effected in such a way that emf in the middle of the current gap should have a zero passage. In that case the control value for regulation is either the frequency with which the stator field is switched or the amplitude of the stator current. Such a method of regulating synchronous motors is described for example in detail in "Sensorless Speed Controlled Brushless DC Drive using the TMS320C242 DSP Controller" by P. Voultoury, Intelligent Motion, May 1998 Proceedings, pages 169-180.
At certain rotary speeds as are required for example in the case of synchronous motors for washing machines or dryers, the use of a gappy stator current however involves undesirable clicking or chattering which is generally not acceptable to a customer. That noise is evidently caused by the fact that the stator windings are acted upon in a pulse-like manner by the pulses of the gappy current, in which case the frequencies which occur here are in the audible range.
It is therefor already known for troublesome noises of that kind to be avoided in the case of synchronous motors by a procedure whereby, in those rotary speed ranges, instead of the gappy current, a sinusoidal or quasi-sinusoidal stator current is used. A quasi-sinusoidal stator current of that kind is produced by the power switches of the three-phase bridge of synchronous motor being operated with pulses which are controlled in pulse width modulated (PWM) manner in such a way that a quasi-sinusoidal stator current is produced. Production of the quasi-sinusoidal stator current by PWM-actuation is described in greater detail for example in "Digitale Steuerung eines Dreiphasen-Induktionsmotors" ("Digital Control of a Three-Phase Induction Motor") by B. Maurice et al in Design & Electronik 8 of 07.04.1992, pages 40-46. In this case the control circuit has recourse to stored tables with values for the pulse duty factors of the bridge arms of the synchronous motor.
Due to the use of a quasi-sinusoidal stator current however it is no longer possible to measure the emf induced in the stator windings and to use the measurement result for regulation of the synchronous motor, as was the case when using the gappy current.